Cycloaddition Reactions Of Cyclopropenones. 4. Reaction Of Some 2-aminopyridines With Methylphenylcyclopropenone

Methylphenylcyclopropenone (2) undergoes a cycloaddition reaction across the PhC-CO bond with 2-aminopyridine (3a) in ether to produce the insoluble cis-3,4-dihydro-3-methyl-4-phenyl-2H-pyrido[1,2-a]pyrimidin-2-one (6a). Rearrangement of 6a to α-methyl-trans-cinnamamide 7a occurs slowly in refluxing chloroform. 2-Amino-3-methyl- and 2-amino-4-methylpyridine react similarly, while 2-amino-5-methylpyridine eventually affords, in addition, the α-(2-pyridylamino) acid 10a. 2-Amino-6-methylpyridine reacts very slowly to give 10b exclusively. A lack of reactivity shown by aniline suggests that both products arise by way of initial nucleophilic attack of the aminopyridine ring nitrogen on the cyclopropenone ring, with steric hindrance at this nitrogen favoring subsequent formation of an intermediate of apparent reaction at the exo nitrogen.41223546354